The present invention relates to a method of mass spectrometry and a mass spectrometer. The preferred embodiment relates to a method of digitising signals output from an Analogue to Digital Converter and determining the arrival time and intensity of ions arriving at an ion detector.
It is known to use Time to Digital Converters (“TDC”) and Analogue to Digital Converters (“ADC”) as part of data recording electronics for many analytical instruments including Time of Flight mass spectrometers.
Time of Flight instruments incorporating Time to Digital Converters are known wherein signals resulting from ions arriving at an ion detector are recorded. Signals which satisfy defined detection criteria are recorded as a single binary value and are associated with a particular arrival time relative to a trigger event. A fixed amplitude threshold may be used to trigger recording of an ion arrival event. Ion arrival events which are subsequently recorded resulting from subsequent trigger events are combined to form a histogram of ion arrival events. The histogram of ion arrival events is then presented as a spectrum for further-processing. Time to Digital Converters have the advantage of being able to detect relatively weak signals so long as the probability of multiple ions arriving at the ion detector in close temporal proximity remains relatively low. One disadvantage of Time to Digital Converters is that once an ion arrival event has been recorded then there is a significant time interval or dead-time following the ion arrival event during which time no further ion arrival events can be recorded.
Another important disadvantage of Time to Digital Converters is that they are unable to distinguish between a signal resulting from the arrival of a single ion at the ion detector and a signal resulting from the simultaneous arrival of multiple ions at the ion detector. This is due to the fact that the signal will only cross the threshold once, irrespective of whether a single ion arrived at the ion detector or whether multiple ions arrived simultaneously at the ion detector. Both situations will result in only a single ion arrival event being recorded.
At relatively high signal intensities the above mentioned disadvantages coupled with the problem of dead-time effects will result in a significant number of ion arrival events failing to be recorded and/or an incorrect number of ions being recorded. This will result in an inaccurate representation of the signal intensity and an inaccurate measurement of the ion arrival time.
These effects have the result of limiting the dynamic range of the ion detector system.
Time of Flight instruments which incorporate Analogue to Digital Converters are known. An Analogue to Digital Converter is arranged to digitise signals resulting from ions arriving at an ion detector relative to a trigger event. The digitised signals resulting from subsequent trigger events are summed or averaged to produce a spectrum for further processing. A known signal averager is capable of digitising the output from ion detector electronics at a frequency of 3-6 GHz with an eight or ten bit intensity resolution.
One advantage of using an Analogue to Digital Converter as part of an ion detector system is that multiple ions which arrive substantially simultaneously at an ion detector and at relatively high signal intensities can be recorded without the ion detector suffering from distortion or saturation effects. However, the detection of low intensity signals is generally limited by electronic noise from the digitiser electronics, the ion detector and the amplifier system. The problem of electronic noise also effectively limits the dynamic range of the ion detector system.
Another disadvantage of using an Analogue to Digital Converter as part of an ion detector system (as opposed to using a Time to Digital Converter as part of the ion detector system) is that the analogue width of the signal generated by an ion arriving at the ion detector adds to the width of the ion arrival envelope for a particular mass to charge value in the final time of flight spectrum. In the case of a Time to Digital Converter, only ion arrival times are recorded and hence the width of peaks in the final spectrum is determined only by the spatial and energy focusing characteristics of the Time of Flight analyser and by timing jitter associated with TDC trigger signals and signal discriminator characteristics. For a state of the art Time of Flight detector the analogue width of the signal generated by a single ion is between 0.4-3 ns FWHM.
Recent improvements in the speed of digital processing devices have allowed the production of ion detection systems which seek to exploit the various different advantageous features of both Time to Digital Converter systems and Analogue to Digital Converter systems. Digitised transient signals are converted into arrival time and intensity pairs. The arrival time and intensity pairs from each transient are combined over a scan period into a mass spectrum. Examples of such systems are disclosed in WO2007/138338, WO2008/142418 and WO2008/139193. Each mass spectrum may comprise tens of thousands of transients. The resulting spectrum has the advantage in terms of resolution of a Time to Digital Converter system (i.e. the analogue peak width of an ion arrival does not contribute significantly to the final peak width of the spectrum). Furthermore, the system is able to record signal intensities which result from multiple simultaneous ion arrival events of the Analogue to Digital Converter. In addition, discrimination against electronic noise during detection of the individual time or mass intensity pairs virtually eliminates any electronic noise which would otherwise be present in the averaged data thereby increasing the dynamic range.
In the known methods, conversion of digitised transient signals into ion arrival time intensity pairs may involve subtraction of baseline, thresholding of data and/or application of Finite Impulse Response (“FIR”) filters to all or part of the digitised signal. The aim of these processes is to reject electronic noise, locate positions within the data corresponding to ion arrival response and determine an ion arrival time and intensity associated with each ion arrival response.
As described above, each ion arrival has an associated analogue peak width. If two or more ions arrive simultaneously then these analogue peak widths may partially overlap making it impossible for a simple Finite Impulse Response filter, peak maxima or related peak detection method to isolate the arrival time and intensity of the individual ions. In such a case a response related to the average ion arrival time and summed area may be recorded rather than two individual ion arrival times an intensities. This coalescing of two or more ion arrivals within a transient into a single time intensity pair can cause artifacts in the final summed data. Furthermore, the analogue peak width from ions of different mass to charge ratio species may overlap significantly within a single transient. This will result in an inaccurate representation of the signal intensity and an inaccurate measurement of the ion arrival time for each mass to charge ratio species.
It is therefore desired to provide an improved ion detector system and an improved method of detecting ions.